1. Field of the Invention
This invention relates in general to a thermal transfer material and more particularly to a thermal transfer material with an excellent back coating layer which keeps the base film from fusing to the thermal head.
2. The Prior Art
For thermal transfer materials, the sublimation type and the heat-melt type have been conventionally known. In the former case, a dye layer comprising a heat sublimation type dye(s) and a heat resistant binder resin(s) is provided on a base such as a polyester film and, by a thermal printing operation from its back side using a thermal head, heat sublimation type images are transferred onto an image forming layer provided on a transfer-receiving base such as paper. In the latter case, a pigment layer comprising a pigment(s) and/or a dye(s) and a heat-melting binder resin(s) is formed and heat-melting images are transferred by means of thermal printing. For such base films for thermal transfer materials, polyester and, for a general purposes, polyethylene terephthalate have been used. However, since they are thermo-plastic, sometimes the thermal head and the base film are fused during thermal printing and the operation becomes impossible.
To solve this problem, the addition of a back coating layer (a back smooth layer) was proposed and disclosed in Japanese unexamined patent publication Tokkai Sho 55-7467, Tokkai Sho 60-225777, Tokkai Sho 62-1575, Tokkai Hei 2-148395, Tokkai Hei 3-61087, Japanese examined patent publication Tokko Hei 4-17160, etc. However, although thermal transfer printers have been improved and their performance has been enhanced, there are problems regarding contamination and wear of the thermal heads. Particularly for the sublimation type, which requires several times more thermal energy than the heat-melt type, even now the heat resistance of the thermal transfer materials is not satisfactory. Even for the heat-melt type, an improvement in heat resistance is desired in response to high speed printing and diversified transfer-receiving bases.
On the other hand, Tokkai Hei 5-85070 proposes a back coating layer(s) comprising a resin(s) prepared by the graft-bonding of organopolysiloxane or a cured product thereof. It features superior heat resistance, film properties and slip properties. However, it does not simultaneously satisfy the heat resistance, slip properties and friction resistance requirements.
The conventional technology as described above uses a thermo-plastic resin for the base and insufficient slip properties and the occurrence of 5locking under harsh thermal transfer conditions have been observed. There is a method in which the resin becomes a cured product by cross-linking the hydroxyl groups and such for the purpose of improving heat resistance. However, even with this method, it is relatively difficult to prepare the resin to be cross-linked with a sufficient amount of hydroxyl groups.